UBC Theses and Dissertations
Electrophysiological investigation of auditory temporal processing MacDonald, Katie Mary
Infant hearing-health programs have a goal of identifying infants with a permanent hearing loss by the age of three months and treating these infants by the age of six months. However, deficits in hearing thresholds are not the only deficits that exist in the auditory system. The ability of an infant's auditory system to resolve rapid changes in acoustic signals (i.e., temporal resolution) and integrate acoustic information over time (i.e., temporal integration) is important for typical language development. Because behavioural responses are unreliable for diagnostic purposes before the age of six months, electrophysiological measures of temporal resolution and integration could be beneficial. The main objective of my thesis was to validate in adults if 80-Hz auditory steady-state responses (ASSRs, an objective electrophysiological measure) can be used to assess temporal resolution and integration. Physiological temporal resolutions of adults were estimated from cortical auditory evoked potentials (CAEPs) and ASSR resets evoked by gaps (1.5625, 3.125, 6.25, 12.5, and 25 ms) within 40-Hz and 80-Hz amplitude-modulated white-noise bursts. Physiological gap-detection thresholds for CAEPs (8 ± 6 ms, averaged across conditions), 40-Hz ASSR resets (6 ± 5 ms), and 80-Hz ASSR resets (5 ± 4 ms) were comparable to behavioural gap-detection thresholds (5 ± 2 ms). However, 40- and 80-Hz ASSRs maximally reset to half-cycle gap durations (i.e. 12.5 and 6.25 ms respectively), thus ASSR resets might not be truly measuring gap-detection thresholds. Conflicting results from CAEPs and ASSR resets to gaps provides evidence that CAEPs respond preferentially to all gaps down to threshold; whereas, ASSRs preferentially reset to gaps that violate their modulation periodicity. Physiological integration times (117 ± 48 ms, averaged across conditions), as measured from the rise time of the ASSR resets, were comparable to behavioural measurements of temporal integration (132 ± 83 ms). However, more research is required to determine if physiological and behavioural integration times are correlated or are coincidentally similar. These results indicate that CAEPs are accurate measures of temporal resolution. However, further research is required to determine the utility of ASSR resets in assessing temporal resolution and integration.
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